1. Field of the Invention
The invention relates to methods for making microcapsules, encapsulating pharmaceutical compounds in microcapsules, microcapsules, microcapsule encapsulated pharmaceutical compositions and products, and methods of using these compositions.
2. Description of the Related Art
Many cytotoxic or bioactive drugs and enzymes cannot be injected intravenously. Others can be injected, but are rapidly degraded before they reach the target tissue. Still others are cleared from the blood by the liver or kidneys so quickly that their biological half-life is too short to be of therapeutic value. Still other drugs are insoluble in aqueous solutions. Since intravenous injection in hydrocarbon solvents is not well tolerated by patients, such drugs are difficult to administer.
One method for overcoming these limitations is encapsulation into microcapsules or liposomes. Encapsulation of drugs or biological therapeutics into liposomes or liquid microcapsules can enable delivery to target organs where the bioactive drug can be released directly by diffusion. Properly designed microcapsules can provide unique methods of direct delivery by parenteral injection, nasal inhalation and dermal administration for sustained release of important bioactive drugs.
Major difficulties with commercial preparation of microcapsules arise when density-driven phase separation of the immiscible carrier fluids occurs. This is especially true when the microcapsules are constructed by forming water/oil emulsions or when attempts are made to encapsulate multiple drugs. This limits the yield and often results in microcapsules that are not spherical nor uniform in size. Non-conformity limits the packing density (and, thereby, the drug payload delivered) when the microcapsules arrive at the target tissues.
Certain current methods of forming microcapsules (such as liposomes) are based on chemical characteristics of certain phospholipids that self-assemble into bilayers when dispersed in an excess of water. Most liposomes carry pharmaceuticals dissolved in the entrapped water phase. Drugs that are insoluble or that have only limited solubility in aqueous solvents pose problems for incorporating into liposomes. Such organic-soluble drugs are usually limited in liposomal formulations to those that bind inside the hydrophobic region of the liposome bilayer. Some drugs are so insoluble that they do not associate with the bilayer and, therefore, have very low encapsulation efficiencies. Certain liposomal drug formulations, including anti-tumor liposomes containing doxorubicin [Gabizion et al. 1992] or muramyltripeptide have been studied extensively in clinical trials.
Microcapsule formation by liquid-liquid dispersion of aqueous drugs and organic solvents typically produces water-in-oil, (W/O) type liposomes. A second requisite step is removal of the organic solvent (typically evaporated) to form reverse-phase evaporation vesicles (rev) or stable plurilamellar vesicles (splv).
Spherical multilamellar vesicles (mlv) are rarely formed by these methods and the size distribution is quite heterogeneous. Typically, in order to generate multilamellar vesicles, film casting techniques with organic solvents, hydration and sizing using filtration through inert membrane filters are required [Talsma and Crommelin 1992]. Methods of forming multi-layered microcapsules often require emulsification of the aqueous phase into organic carrier solutions by shear, bubbling or sonication. Sophisticated, multi-step emulsion technology is required and yields of uniform type and size are often very low.
Liquid microemulsions also have been developed as drug delivery systems, especially for drugs that are poorly soluble in aqueous carriers. A microemulsion typically contains droplets in the range of 0.1-1 .mu. in diameter. Such microemulsions are characterized by very fluid and dynamic micelles which are formed by sequential mixing of one immiscible phase with another using surfactants and co-surfactants [Bhargava et al. 1987]. Typically, surfactants that produce water-in-oil (W/O) microemulsions have a hydrophilic-lipophilic balance (HLB) rating of 3 to 6, while those that produce oil-in-water (O/W) microemulsions have an HLB of 8 to 18. The surfactants can be non-ionic, ionic, or amphoteric. Often, medium chain-length alcohols are added as the co-surfactant in the last step in achieving the final microemulsion.
A disadvantage of microemulsions is that each micelle (liquid capsule) is too small (typically, less than 1.0 micron) for deposition in larger vascular beds when administered by intravascular injection. Therefore, microemulsions are not suitable for chemoembolization type treatment of vascularized tumors. Additionally, since microemulsions are true colloidal suspensions, they cannot be scaled up to large enough size for many intravascular drug delivery applications. Microemulsions formed with lipid soluble anti-tumor agents and low density lipoproteins (LDLS) have been used to target drugs to neoplastic cells that require large amounts of cholesterol for synthesis of cell membranes [Halbert et al. 1984]. However, LDLs also attract phagocytes making the amount of drug actually delivered to the tumors and thence the therapeutic dose difficult to determine.
The use of solid matrix microspheres containing adsorbed drugs within the matrix is also known. For instance, U.S. Pat. No. 4,492,720 to Mosier disclosed methods for making microspheres to deliver chemotherapeutic drugs (including Cis-Platinum) to vascularized tumors. This method of preparing microspheres is accomplished by liquid encapsulation and solid-phase entrapment wherein the water-soluble drug is dispersed in a solid matrix material. The method involves dissolving the aqueous drug and the matrix material in an organic solvent, in which they are mutually soluble, then dispersing this mixture in a second organic solvent to form an emulsion that is stable enough for intravascular injection.
Other approaches have utilized copolymers such as polyvinyl chloride/acrylonitrile dissolved initially in organic solvents to form microcapsules containing, for instance aqueous enzyme solutions. U.S. Pat. No.3,639,306 to Sternberg et al. discloses a method of making anisotropic polymer particles having a sponge-like inner support structure comprising large and small void spaces and an outer, microporous polymer film barrier. A multiple-step batch process is used which entails removal of the organic solvents used to dissolve the polymers prior to addition of aqueous components. Solid-matrix microspheres, however, are often not perfect spheres thereby limiting the packing density. Additionally, many drugs cannot be trapped or adsorbed in these systems at effective concentrations and drug-release rates are often not constant.
Conventional methods of forming multi-lamellar, immiscible, liquid microcapsules are limited, because of density-driven phase separation and stratification into horizontal layers resulting in the necessity to use multi-step, batch processing including mechanical mixing and solvent evaporation phases [Talsma and Crommelin 1992]. Each batch step suffers losses which reduce overall efficiencies. Conventional solvent evaporation methods do not permit simultaneous formation of the outer skin as the microcapsule itself is formed. Many conventional therapeutic microcapsules or liposomes have natural phospholipid outer skins (usually in combination with cholesterol and a fatty amine such as stearylamine) and therefore are subject to elimination by immune cells. Other conventional methods use sialic acid and other coatings on the lipid bilayer to mask the liposomes from detection by the scavenging systems of the body. Without an adequate outer skin, microcapsules often coalesce thereby reducing shelf-life.
For instance, U.S. Pat. No. 4,855,090 to Wallach, discloses a method of making a multilamellar lipid vesicle by blending an aqueous phase and a nonaqueous lipophilic phase using a high shear producing apparatus. The lipophilic phase is maintained at a high temperature (above the melting point of the lipid components) and is combined with an excess of the aqueous phase, which is also maintained at a high temperature. U.S. Pat. No. 5,032,457 to Wallach discloses a paucilamellar lipid vesicle and method of making paucilamellar lipid vesicles (PLV). The method comprises combining a nonaqueous lipophilic phase with an aqueous phase at high temperatures and high shear mixing conditions, wherein the PLVs are rapidly formed in a single step process. These methods do not, however, include internal mixing of immiscible phases inside the PLV's after they are formed.
U.S. Pat. No. 4,501,728 to Geho et al. discloses the encapsulation of one or more drugs or other substances within a liposome covered with a sialic acid residue for masking the surface of the membrane from scavenging cells of the body utilizing techniques known for the production of liposomes. In one embodiment, additional tissue specific constituents are added to the surface of the liposome which cause the liposome thusly treated to be attracted to specific tissues. Similarly, U.S. Pat. No. 5,013,556 to Woodle et al. provided methods for making liposomes with enhanced circulation times. Liposomes created by this method contain 1-20 mole% of an amphipathic lipid derivatized with a polyalkylether (such as phosphatidyl ethanolamine derivatized with polyethyleneglycol). U.S. Pat. No. 5,225,212 to Martin et al. discloses a liposome composition for extended release of a therapeutic compound into the bloodstream, the liposomes being composed of vesicle-forming lipids derivatized with a hydrophilic polymer, wherein the liposome composition is used for extending the period of release of a therapeutic compound such as a polypeptide, injected within the body. Formulations of "stealth" liposomes have been made with lipids that are less detectable by immune cells in an attempt to avoid phagocytosis [Allen et al. 1992]. Still other modifications of lipids (i.e., neutral glycolipids) may be affected in order to produce anti-viral formulations. U.S. Pat. No. 5,192,551 to Willoughby et al. 1993. However, new types of liposomes and/or microcapsules are needed to exploit the various unique applications of this type of drug delivery.
Liquid microcapsules and liposomes can provide effective drug delivery by intravascular injection, nasal inhalation and dermal administration for sustained release of bioactive chemicals. However, drug delivery of bioactive drugs, or enzymes or biocatalysts entrapped in liquid microcapsules and liposomes are limited to those biochemicals whose useful biological shelf-life typically is more than a year. Many bioactive drugs possess chemical or bioactive half-lives which last only days. Microencapsulation has been used to greatly extend the normal two-hour half-life of labile enzymes to prolong the effective half-life up to 70 hours after injection (Chang, 1971). However in that instance, the active drug is encapsulated, rather than the inactive prodrug form, since it is difficult to chemically alter a drug once it has been encapsulated. Short-lived drugs could be effectively delivered inside microcapsules if a method of encapsulating a long-lived precursor drug form can be combined with a method for the in situ conversion of the precursor to the short-lived active form, just prior to, or even after administration.
Controlled release of drugs from liposomes has been achieved by using temperature sensitive polymers in the formation of the liposomes (Magin et al. 1986). Once the liposomes are localized in the target tissue (or tumor) the drug can be rapidly released if the local tissue temperature can be raised above the transition temperature of the liposome membrane. This requires some method of controlled tissue heating which is difficult to achieve without complicated surgical procedures, implanted interstitial antennas or ultrasonics to produce effective local hyperthermia (Hand, 1991). However, there is no method of activating a prodrug or predrug in a liposome using electromagnetic energy and/or ultrasound.
In a controlled delivery system described by Supersaxo et al. in U.S. Pat. No. 5,470,582, pore-containing microcapsules are preformed of polymers such as polyesters, polyamides, polyanhydrides and polyacrylates and an active agent is allowed to migrate into the microcapsules through the pores. After administration, the active agent is released through the pores by diffusion. A burst of release may be caused by application of ultrasonic radiation. Another system, described by Mathiowitz et al. in U.S. Pat. No. 4,898,734, is also based on passive or facilitated diffusion of an active agent from pore-containing polymer microcapsules. Methods of facilitating diffusion include exposure to high temperature, light, or ultrasound. This patent also describes degradable microcapsules and microspheres immobilized in a polymer matrix. A controlled release delivery system described by Modi in U.S. Pat. No. 5,417,982 is biodegradable copolymer based microcapsules in which delayed release of an active agent is controlled by the time required for enzymatic digestion of the polymer matrix. Wheatley et al. describe in U.S. Pat. No. 4,933,185, microcapsules having an inner core and an outer, ionic skin. An active agent and an enzyme are encapsulated in the inner core, such that the enzyme degrades the inner core and releases the active agent. All the carrier systems described in this paragraph encapsulate the active agent rather than a prodrug, and thus do not overcome the limitations of administering short-lived active agents as described above.
A method of in situ activation of a prodrug is described in U.S. Pat. No. 5,433,955, issued to Bredehorst et al. This method is a two step process in which an activator bound to a targeting moiety is administered to a subject. In a second step, the prodrug is released into the circulation and becomes activated only where the activator is bound. A disadvantage of this method is, because neither the activator nor the drug are encapsulated or enclosed in a carrier, the activator substance is exposed to the serum of a subject and may contact a substrate prior to reaching its target site, thus causing possible side effects. In addition, unbound activator must be cleared from the system prior to administration of the second agent. Also care must be taken to avoid immunological reactions to both the activator and the prodrug.
It is evident, therefore, that improvements are still needed that address certain drawbacks of conventional liposome or microcapsule delivery systems as well as those of in situ drug activation. For example, conventional liposomes or microcapsule delivery systems are only useful for drugs with a long shelf life or biological activity. If the bioactive form of the drug is short lived or chemically labile, the effective shelf life of the encapsulated drug may make it impractical for normal pharmaceutical storage. In addition, delivery of an active drug to a specific site in the body with a liposome formulation still presents difficulties. There is a need for a type of microcapsule delivery system that is stable for long term storage and that preferably would be able to contain both a drug precursor and the activator for that precursor so that activation of the precursor to its active form can take place just prior to administration, or even after administration and confirmation that the drug delivery system (microcapsules) is in the desired location. Even more advantageous would be the ability to specifically activate only that portion of the delivery system that is in the desired location. Finally, there is a need to be able to accomplish all that without physiological changes or damage in the surrounding healthy tissue.